PURE AND SIMPLE: SEM-EBSD ANALYSIS OF QUARTZITE CPOS FROM THE RUBY-EAST HUMBOLDT EXTENSIONAL SHEAR ZONE, ELKO COUNTY, NEVADA

Quartzites along a transect parallel to mylonitic stretching lineation in the Ruby-East Humboldt (R-EH) extensional shear zone preserve well-developed and systematic crystallographic preferred orientations (CPOs) dominated by c-axis maxima parallel or close to the Y strain axis and defining WNW-asymmetric girdles consistent with diverse kinematic indicators and previous reports. Less commonly, modes associated with basal, steep prism or (in one case) prism<c> slip are also observed. The R-EH shear zone formed and evolved under high amphibolite conditions, and grain boundary migration may have enhanced CPOs as grains configured for easy-slip replaced those in “harder” orientations. Misorientation analysis indicates the dominance of prism<a> slip, possibly operating in tandem with rhomb<a> slip in the off-axis modes. C-axis maxima commonly form more than one mode, with widespread Dauphiné twinning explaining much but not all of the bimodality. For example, modes parallel to Y commonly place one prism either sub-parallel or nearly normal to the inferred instantaneous shortening axis, thus disposing the other two prisms for antithetic slip. In general, a-axis maxima do not align with lineation as predicted for end-member simple shear. Rather, deviation into the subsimple shear regime activated antithetic slip systems whose competing vorticities served to drive the CPOs toward a few stable end orientations. The principal stresses must have been compatible with operation of all active slip systems, and thus these antithetic pairings limit the position of the instantaneous shortening direction and severely restrict the range of possible vorticities. Kinematic vorticity in the shear zone was generally high, but the departure from end member simple shear is nonetheless important. Strain compatibility requires that if either or both walls of the shear zone were stretching, then the shear zone itself must have stretched, with profound implications for crustal-scale kinematics. If the pure shear contribution is distributed throughout the deeper crust whereas simple shear is focused into narrow zones, then the shear zones will exhibit strong noncoaxiality even if pure shear regulates the deep-crustal strain path.